1. Field of the Invention
This invention relates generally to the improved sealing of plastic containers and, more specifically, to the addition of fillets to a zipper closure for plastic bags and similar containers in order to eliminate gaps formed between the two sides of the zipper closure which allow for leakage of air or liquid, particularly at the side seals of such containers.
2. Description of the Prior Art
Zipper closures have long been used to improve the sealing of, and simplify the closing of, plastic bags and similar containers. Plastic bags having zipper closures typically consist of two substantially similar-sized sheets of plastic film (usually supplied from a pair of continuous web spools or rolls) which are sealed together at a lower end of the sheets to form a front layer and a rear layer, with the seal forming a bottom edge of the bag (or, alternatively, the plastic bag may be formed by a length of bag film folded over upon itself to form a front layer and a rear layer connected by an integral bottom edge defined by the fold); and two opposing lengths of plastic film heat sealed along the inside of the upper edges of the front and rear layers of bag film, with each of the lengths of plastic film carrying two or more interdependent ridges. The lengths of film appear interdigitated in cross-section due to these interdependent ridges which form the zipper closure. Side edges of the plastic bag are typically sealed using a sealing head.
It is well-known that the zipper closure itself is air-tight along its length due to the releasable and reusable seal formed by the interdependent ridges. A long-standing problem in the art of zipper-sealed plastic bags or similar containers, however, has been the presence of escape gaps which are created during heat sealing at the outermost ridges (i.e. such as at the bag's two side seal locations) of the zipper closure. These undesirable gaps as formed by conventional in-line web type manufacturing of bags allow for air and/or liquid to leak into or out of the sides of a plastic bag. One major contributing factor causing the escape gaps is the abrupt change in profile between the ridge portions of one of the layers of the zipper closure and the relatively flat extensions of the opposite layer of the zipper closure. The relatively flat extensions are preferred, in that they provide convenient portions of plastic used to heat seal the respective layers of the zipper closer to the front and rear layers of the bag film.
Most problematic are the escape gaps located at the intersection of the zipper closure, the bag film, and the side edges of the plastic bag. These escape gaps are frequently formed as a result of applying continuous pressure and heat to seal the side edges of the bag, without making accommodations for the underlying zipper closure that is typically already heat sealed along the top edge of the bag film. Such escape gaps facilitate leakage of air and/or liquid directly into or out of the contents of the plastic bag, which can cause undesired spillage, contamination, and spoilage of such contents.
U.S. Pat. No. 3,986,914, issued to Howard, discloses one method for eliminating the escape gaps at the side edges of the bags, consisting of forming a bead seal at the junction of each outermost ridge of the zipper closure and the side edges of the plastic bag. The patent discloses forming the bead seal during heat-welding of the side edges of the plastic bag. The bead seal is made of plastic that is forced into the junctions during heating of the container and zipper closure by an apparatus called a pressure bar. The specially configured pressure bar includes a U-shaped indentation or channel, disposed so that the walls of the channel straddle and slightly pinch the zipper closure when the pressure bar and a cooperating surface (such as a sealing bar or an anvil) are in contact with one another. The U-shaped channel within the pressure bar provides a pressure differential which causes heated plastic to flow into the junctions, thus forming the bead seals.
One shortcoming of the method for making bead seals described in Howard (U.S. Pat. No. 3,986,914) is that the bead seals are formed as a separate step in the manufacturing process. Also, the pressure bar with the specially configured channel is not found on conventional bag-making machinery, but rather, manufacturing plants would need to be retrofitted with such pressure bars, thus incurring at least some additional cost, which may eventually have to be borne by consumers as an increase in the price of plastic bags. Another shortcoming of such method is that the U-shaped channel within the specially configured pressure bar must hit the zipper closure in a precise orientation each time it contacts the zipper closure. However, plastic film is difficult to keep in a proper orientation, particularly during an in-line web-type manufacturing process, wherein as the plastic film moves downstream it has a tendency to wander from side-to-side. As the U-shaped channel of the pressure bar disclosed in Howard contacts the plastic film, the relative orientation is hard to keep constant due to the side-to-side movement of the film. As a result, if the pressure bar misses the precise location of the zipper closure on a film web by merely a fraction of an inch, many of the resulting plastic bags must be rejected as unusable when made in the prior art process as disclosed by Howard. The present inventor believes that this drawback of Howard is why, despite that patent being issued more than twenty years ago, no manufacturers in the plastic bag-making industry are believed to be currently using the Howard process.
Yet another drawback of the method disclosed in the Howard patent is that there is only inferior means for sealing the zipper ends together. If the zipper ends are not adequately sealed together, they will leak. The Howard method requires use of high, concentrated pressure in order to seal the ends of the zipper, which is known in the art as "smashing" the zipper, and which does not always create an adequate seal at the ends of the zipper closure.
Another practical consideration that makes the Howard process inferior is that, although the process does attempt to reduce escape gaps, it does so by deforming the actual sealing profile of the zipper closure. By borrowing material from the interlocking portions of the zipper closure to close escape gaps, the Howard process undesirably compromises the integrity of the zipper seal. Thus, although plastic bags made by the Howard process may be more leak-resistant (i.e. more gas-tight and liquid-tight) at rest than those bags made by other conventional techniques that did not eliminate escape gaps, such bags made by the Howard process would tend to open prematurely when subjected to even minor forces, for example when the contents of a plastic bag falls against the zipper closure.
Another conventional attempt of increasing the leak-resistance of plastic bags having zipper closures has been to preheat the areas where the zipper closure meets the side edges of the plastic bag. This prior art technique is demonstrated in FIGS. 1 and 2 of the present application. FIG. 1 shows an enlarged cross-section, rotated 90.degree. for convenience, of a conventional two-part zipper closure member 11 taken along a side edge of a plastic bag 10, just after the bag-making stages wherein the two parts of the zipper closure 12, 16 are respectively heat sealed to the front and rear layers 14, 18 of the bag, and before the sides of the two layers of the zipper closure are melted together (i.e., at the extreme side edges of the plastic bag 10). The front part of the zipper closure bearing reference number 12 is adjacent to the front layer 14 of the plastic bag, and the rear part of the zipper closure, bearing reference number 16, is adjacent to the rear layer 18 of the plastic bag.
FIG. 2 demonstrates the problem of escape gaps present in prior art devices which form in part because the melting of the sides of the zipper closure is typically uneven and cannot be relied upon to completely eliminate escape gaps at the outer ridges of the zipper closure. As a result, air and liquid can still leak out the sides of the plastic bag at the intersection of the bag's side edges and the zipper closure. During the side edge sealing step, which consists of exposing the sides of the bag 10 to a sealing head, the front layer 14 and rear layer 18 of the bag 10 below the zipper closure are sealed together along a seam designated by reference number 20. By preheating the sides of the layers 12, 16 of the zipper closure prior to exposing the bag 10 to the sealing head, the relatively flat portions 30, 31 and 32, 33 of the zipper closure layers melt together during the side edge sealing step along melt lines 22 and 24. However, as shown in FIG. 2 and due in part to the abrupt change in profile between the ridge portions 26, 28 of the rear zipper closure layer and the flat portions 30, 32 of the front zipper closure layer, escape gaps 34, 36 form, thus allowing leakage of air and liquid through the zipper closure at the side edges of the plastic bag 10.
One object of the present invention is therefore to eliminate formation of the undesirable escape gaps between the ridges of the zipper closure by providing a means for making the change in profile between the ridge portion of the zipper closure and the rest of the bag film more gradual. Another object of the present invention is to provide zipper closures for plastic bags and similar containers that are air-tight and liquid-tight, even along their side edges.
Yet another object is to reduce the number of rejected, unusable plastic bags from the number of rejects produced by manufacturing processes of the prior art. An additional object of the present invention is to provide a zipper-type plastic bag manufacturing process suitable to practice using existing, conventional heat dies in an in-line web-type process, such that there is no need for the use of a pressure differential-producing die to impart any special profiles to the zipper closure of the plastic bags. The manner in which these and other objects of the invention are accomplished will become clear from the Summary of the Invention, the Detailed Description of the Preferred Embodiments, and the accompanying drawings.